Police are searching for a group of people who have been posing as police officers and pulling motorists over to search their cars for alcohol.

Northland police said a group of four, travelling in a red Holden Commodore on Sunday, pulled a woman over on Otaika Road, near the Whangarei information centre, with flashing red and blue lights.

It is apparently the second such incident this month.

Police said the first incident occurred in the early hours of Saturday, February 9, but that vehicle was described as a red Mazda.

Four men in plain clothes then approached the woman and flashed an ID, saying they needed to search her car boot for alcohol because she was travelling in a liquor ban area.

Police said one man asked the woman for her ID and also asked how much money she had in her possession.

When she questioned them, they said they had another job to do and sped off.

Whangarei Area Commander Tracy Phillips said if people thought they were being pulled over by an imposter, they should not get out of the car, but should challenge them on their identity.

"Mostly it will be a uniformed officer pulling people over, but on occasion plain clothes officers do.

"There are a few ways to tell, even if they are plain clothes they will most likely be wearing a stab-proof vest and will generally always have a utility belt on."

She said a plain clothes officer especially should introduce themselves and show their ID.

If they didn't, a person was more than entitled to ask to see it.

"There should be the officer's name, a picture of the officer and an ID number. You can also ask them to wait while you call the 111 line and the communications operator will be able to tell you straight away whether that officer is actually on shift or not."

Phillips said it was lucky nobody's personal safety had been directly threatened by the imposters.

"But as people get a bit bolder they do more and more stupid things."

The maximum term for impersonating a police officer was 12 months imprisonment.

Anyone with information should call Whangarei Police on 09 430 4500 or the anonymous Crimestoppers line on 0800 111 555.

Red and blue flashing lights, similar to those found on the dashes of police plain clothes cars, can be purchased on Trademe for as little as $45.